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Joshua Tree National Park, A Plant Lovers Paradise

A Visit to the Desert

One of the things I enjoy the most during the spring time is visiting our nation's desert parks or wilderness areas. Temperatures are moderate and enjoyable, winds usually low with sunny skies, and many species are in bloom. Our desert areas are a treasure trove of plants, often with high densities of different species within a relatively short distance from each other. The desert pavement between plants, absence of tree canopies and xeric plants' limited foliage allows for better viewing of individual plants and plant groupings.

The Desert Community

One such desert is the Mojave Desert, with Joshua Tree National Park located at the southern end of the Mojave, and northwestern end of the Colorado Desert, a subdivision of the Sonoran Desert to the south. The Mojave Desert itself supports 2600 species of plants (and that's not including plants only found above 7500 feet in elevations in the taller mountain ranges). The larger Sonoran Desert hosts over 5000 species and includes all of the worlds biomes within its boundaries – tundra, coniferous forest, temperate deciduous forest, grassland, chaparral, desert, and tropical forest (A Natural History of the Sonoran Desert).

The Mojave Deserts lies along the middle of a chain of North American deserts. At the northern end lies the Great Basin Desert, the second largest and most northern and coldest of the deserts, where sagebrush (Artemisia tridentata) is the dominant plant. Next lies the Mojave, smallest in acreage, the higher in elevation than the Sonoran, and the desert that receives the most rainfall in winter. The Joshua Tree (Yucca brevifolia) is the indicator plant of the Mojave.

And immediately south is the Sonoran Desert, the warmest of the deserts and the most ecologically diverse. The Sonoran Desert is divided into six main subdivisions in the United States and Mexico. The American part of the Sonoran Desert features two subdivisions, the Arizona Upland and Lower California River Valley, also known as the Colorado Desert. The plant we most associate with the Sonoran Desert is the saguaro cactus (Carnegia gigantea), the indicator plant of the Sonoran. Other featured plants in the Arizona upland subdivision of the Sonoran are the organpipe cactus (Stenocereus thurberi) found only close to the border with Mexico, Ocotillo (Fouquieria splendens), Palo verde trees (Parkinsonia/Cercidium) and creosotebush (Larrea tridentata).

The Lower Colorado River Valley subdivison of the Sonoran Desert is lower, hotter and drier than the other subdivisions with summertime temperatures reaching 120º and surface temperatures nearing an astonishing 180º. Saguaros, trees and other vegetation are smaller and more sparse if at all, usually found at or close to water sources and somewhat higher elevations. Communities of creosotebush and white bursage (Ambrosia dumosa) are common throughout.

Because of the variation of the terrain within Joshua Tree National park's two deserts, the Colorado and Mohave Desert, featuring springs, playas, mountains and plains, there is a tremendous diversity of plant life within a relatively small area allowing one to explore areas of many plant alliances and associations. One can easily drive through Joshua Tree National Park (JOTR) in one day and view plant life in the harsh expanse of the Colorado Desert and the slightly higher, cooler and moister Mojave. Cottonwood springs, palm oasis, salt flats, cactus beds, boulder gardens, sand dunes, desert pavement, mountain overlooks and Joshua tree woodlands are some of the plant delights that entice one to exit the car and walk, if only for short hikes. (Desert wash at left with smoketree.)

Another interesting bit of information to know is that of the 790,636 acres within the boundaries of JOTR, 594,502 acres are classified as wilderness under the Wilderness Protection Act of 1964. Wilderness is defined as “an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain. . . . an area of undeveloped Federal land retaining its primeval character and influence, without permanent improvements or human habitation, which is protected and managed so as to preserve its natural conditions. . .” (Wilderness Act). Laws different from laws that govern the use of national parks govern wilderness acreage, limiting roads, signs facilities, etc. within wilderness boundaries. Therefore, over 75% of Joshua Tree National Park acreage is in as much of a natural and native state as possible. Preserved and protected wilderness areas allows all of us the chance to see nature in as an unspoiled state as possible, yet with access (but not convenience). The United States was the first country to designate and protect areas as wilderness.

Typical vegetation of desert biomes can include plants in riparian zones, many woody shrubs and subshrubs, many annual plants and herbaceous perennials, some grasses and few trees. Succulents grow in desert biomes, but will vary in appearance from large fleshy succulent leaves of the Cactaceae family in more moist areas of the desert to stiff, coated leaves of the yucca and nolina. Vines and flowering epiphytes are few to none in desert regions. The mix of vegetation types varies from location to location. (Photo at right taken atop Keys View in the San Bernadino Mountains JOTR.)

The desert biome is “a biological communuity in which most of the indigenous plants and animals are adapted to chronic aridity and periodic, extreme droughts, and in which these conditions are necessary to maintain the community's structure,” (The Natural History of the Sonoran Desert). Over thousands of years, plants formed different communities, alliances, and associations depending on soil type, elevation, moisture, wind, and aspect. Individual plants species, like people, have individual ranges of tolerance to environmental conditions with each growing where they are able (without outside interference). Although many plants have a wide range of adaptability, the factors of plant competition and opportunity (seed spread and availability) also play into the diversity mix of a given area. (Photo at left is in a Chuparosa alliance.)

Vegetation types, or plant communities, are determined by the dominant species within an area, the dominant species being the plant most abundant or occupying the most area. There are many subsets of vegetation types within biomes, often called plant alliances. These alliances are further broken down into associations. (Plant Communities, Joshua Tree Natioanl Park.) Seasonal ephemerals, usually annuals, and geophytes (bulbs and corms), inhabit the various alliances/associations within the desert community. These areas usually do not have hard boundaries, but a blending of plants from one alliance/association to another, referred to as ecotones. Ecotones usually have a higher degree of diversity containing plants from both bordering areas as well as plants not found in either. Perhaps the greater diversity of JOTR is, in part, because of the blending of the Colorado and the Mojave Deserts, if viewed as a large ecotone.

Adaption Mechanisms for Desert Conditions

The low rainfall and high heat of deserts are often thought of as a hostile environment, but in the plants view, it as normal. They have adapted over the longterm to survive under these conditions. Xeric plants use three main adaptive strategies: succulence, drought tolerance and drought evasion either singly or parts of each together. Succulents store water in fleshy leaves, stems or roots and have extensive, shallow root systems to capture water quickly. Most desert rains are brief and light, often wetting only the top few inches of the soil. This allows for capture and storage before evaporation. In deserts, the main succulent group is the cactus. Succulents also have few leaves and are coated to minimize evaporation. Another mechanism some succulents use is the CAM system of photosynthesis, where the stomata are closed during the day and open for transpiration at night. CAM succulents also have the ability to idle their metabolism by keeping the stomata closed both day and night, greatly slowing growth, but can resume growth within 24-48 hours once favorable conditions return.

Drought tolerance is another method of adapting to heat and drought, the ability to withstand dessication. The most visible way in which they accomplish this is called microphylly (growing tiny to small leaves) or dropping their leaves, which reduces transpiration by reducing surface area available for evaporation. Desert shrubs have very far reaching root systems or very deep tap roots. Another method is through yet a different form of photosynthesis called C₄ metabolism, which allows plants to use carbon dioxide more efficienty, thus transpiring less water. Orientating the leaf surface away from the sun (an example is the Jojoba shrub, photo at left), self shading (by tiny hairs on leaves and stems), and waxy coating on leaves are additional methods to reduce evaporation and to cool the plant. (Shrub at left unknown.)

Drought evasion is accomplished by “not existing” during unfavorable periods. Desert annuals, bulbs and corms are examples of plants that live in seed form or underground until such time is favorable for the rest of their life cycle -- growth, flowering and seed production. (Plant Ecology of the Sonoran Desert Region.)

Viewing plants within their communities, alliances and associations helps one to better understand the ecology of the area and conditions necessary for them to thrive. As a gardener, viewing any particular plant in their context helps one to better place them in particular micro-niches of one's own landscape and advises the gardener on better groupings or plant composition of beds and borders. A number of plants have specific environmental demands for completing their life cycle (unaided by gardeners) and occupy smaller acreage. For instance, the smoketree grows within dry washes, wheras other desert trees or shrubs grow along the banks, such as the palo verde grows along the banks of wide washes and the desert lavender inhabits the banks of narrow washes. They each have their own particular, though subtle (to us), micro-niche. Others, such as the creosotebush, grow over many different micro-niche envoronments. Stepping back and looking at the broader community reinforces the knowledge of adaptability and versatility of many plant species.

Joshua Tree National Park Vegetative Survey

In addition to including portions of the Mojave (in the higher and moister and slightly cooler western and northern regions) and Coloradan Desert (regions south and east below 3000 ft.) within Joshua Tree National Park, limited riparian habitats, “sky islands” and California chaparral habitats lie within the park. These three bioregions support nearly 750 documented vascular plant species. Fifty percent of the flora are annual species with summer annuals (that grow in response to summer rains) comprising a full five percent of the total taxa. JOTR nurtures 40 rare protected species and is considered a pristine environment even though there are 50 non-native plant species -- considered to be a small number, including the dreaded salt cedar (Tamarix), Russian thistle, and fountain grass (Pennisetum setaceum) in JOTR.

JOTR receives variable summer (Colorado Desert portion) and winter precipitation (both deserts) with a “southeastward gradient of increasing temperature, decreasing average elevation and decreasing precipitation.” (A Summary of the JOTR Vegetation Mapping Project). The park receives from 5 to 12.5 inches of precipitation annually, depending on the location. (Desert wash at left photo.)

The vegetation of JOTR was classified, described and mapped by the U.S. Geological Survey (USGS) and the National Park Service (NPS) Vegetation Characterization Program, completed in 2012 and the technical report issued in 2013. The 2012 version of the vegetation map for the Park includes a total of 130 map classes, consisting of 51 Alliances, 78 Associations , and eight non-vegetated map classes.

Although Joshua Tree National Park is named after the Joshua Tree yucca, Yucca brevifolia, the largest amount of land cover in JOTR is classified under the Larrea tridentata — Ambrosia dumosa Shrubland Alliance (creosote bush/white bursage), which is divided into seven Associations. This Alliance covers a total of 279,229 acres, representing 35% of the total area. Additionally, the second largest Alliance in terms of land cover is the Larrea tridentata — Encelia farinosa Shrubland Alliance, the creosotebush/brittlebush alliance; it covers a total 174,951 acres, or 22% of the total area and is broken down into three Associations. Thus 57% of JOTR is occupied by just two of the 51 Alliances predominately in the Colorado Desert portion of the park: creosotebush and white bursage, with a good smattering of brittlebush. Knowing this shouldn't dampen ones enthusiasm for the splendid diversity one will find (Summary of the Joshua Tree National Park). (Photo at right is of pancake pear cactus, Opuntia chlorotica, and Parry's nolina, Nolina parryi. in the Muller oak alliance)

Joshua Tree is a national park of diverse terrain and ecosystems, and naturally plants. I've detailed eleven plants that caught my attention during our day trip in mid-March, 2015. Mid-March is a perfect time to visit as the daytime temperatures were in the upper 70's, sunny and calm where we found many plants in bloom. The description and listing of plants in association with them are not complete by any means. Listed are only the most frequent and densely occurring plants. Of paramount help to me in describing a few of the 51 plant alliances was A Summary of the Joshua Tree National Park: Vegetation Mapping Project PDF Report, which maps and details the vegetation of JOTR. Much of the information on the various plant alliances below is derived from that technical report.

Select Plants and Alliances of Joshua Tree National Park

Creosotebush – Last Plant Standing or The Desert Smells Like Rain

A description of the creosotebush is fitting to begin this brief floristic tour of Joshua Tree National Park, as it is ubiquitous. Creosotebush, Larrea tridentata, belongs to the Zygophyllaceae, or caltrop family.

Although one or two alliances make up over 50% of the JOTR, there is interest in viewing the plants within the creosotebush shrubland alliances and associations because of their diverse nature. To be classified as a shrubland, a distribution of the dominant shrub must be present providing an even, if sparse structural component where either the shrub influences the distribution or population dynamic of other plant species or it plays an important role in the ecological process of the stand.

Creosotebush itself is an interesting plant. It is the most drought tolerant perennial in North America. From the high moisture end of its range to a scant 3 inches annually, its plant associations fall from 300 down to just one plant, white bursage, as precipitation decreases. The Larrea tridentata — Ambrosia dumosa Shrubland Alliance (creosotebush scrub) could be considered the last plants standing, in terms of water thriftiness.

The Larrea tridentata — Ambrosia dumosa Shrubland Alliance can be found growing from about 220' – 4200' elevation on small washes, rills, alluvial fans, bajadas, colluvium (sediments deposited due to erosion and runoff) and many aspects of rocky upland slopes in soils that are well drained and sandy, rocky and fine clay. Additionally, creosotebush is found in many other plants alliances, however, not as a dominate member.

Creosotebush is an evergreen shrub usually growing from 2-7 feet tall and wide (and can grow larger), and is widespread in the Mojave, Sonoran, Chihuahuan, and into limited portions of the Great Basin Desert. Creosotebush displays minor differences in shape and chromosome counts in the different deserts, yet still considered the same species. There are four other Larrea species in South America, where it is thought to have originated.

The small dark green resinous and aromatic leaves have a waxy coating that discourage evaporation. The fragrance referred to as the “smell of rain” in the desert is actually the aroma or volatiles emitted from the leaves after a rainfall. It is a pleasant smell. Creosotebush has a long bloom season of pleasantly fragrant yellow flowers from November to May (some references, throughout the year) in Joshua Tree, most abundantly after spring rains.

The roots of creosotebush are said to inhibit the germination and growth of other creosotebushes and others, as they are viewed as being evenly spaced in the landscape. However, current thinking is that their roots are so efficient in using up close available moisture as to deny any other comers a sip. It's been noted, however, that creosotebush acts as a nurse plant and shade canopy for some cactus species.

Creosotebush is one of the longest lived plants; the oldest lived creosotebush ring lives within the center of the Mojave Desert and is estimated to be 11,000 years old. It is said to be cold hardy to only 5º.

Other plants found in differing areas and alliances with creosotebush are desert lavender (Hyptis emoryi), foxtail cactus (Coryphantha alversonii), beatertail cactus (Opuntia basilaris), silver cholla cactus (Cylindropuntia echinocarpa), branched pencil cholla cactus (Cylindropuntia ramosissima), hedgehog cactus (Echinocereus engelmannii), California barrel cactus (Ferocactus cylindraceus), Mormon tea (Ephedra nevadensis), cheesebush (Ambrosia salsola), desert trumpet (Eriogonum inflatum), Parry Rock Pink (Stephanomeria pauciflora), and wishbone bush (Mirabilis laevis var. villosa) among others. A number of these plants have their own plant alliances where they are the dominant species and creosote bush is also present to a lesser or greater extant, depending on the terrain. (Photo at left is the ocotillo, and cactus at right is the cotton top barrel cactus, Echinocactus polycephalus var polycephalus.)

Creosotebush is also called greasewood, however the wood and coal tar product, creosote oil, is not derived from this plant.

Desert Willow

Desert Willow (Chilopsis linearis) trees are found in JOTR primarily in arroyos and wide, low, linear and convex washes in sandy soil from 1000 – 3800'. Desert willow thrives throughout the Colorado Desert, in washes, dry canyons and along streams and rivers of the Sonoran and Chihuahuan Deserts in California, Arizona, Texas, New Mexico and Mexico. (Photo at left taken at Big Bend National Park, and at right from the Amarillo Botanical Garden of the desert willow.)

The desert willow grows in great enough density to form the Chilopsis linearis Woodland Alliance. To be classified a woodland, an areas must have a tree-dominated stand of vegetation with between 25 and 60 percent cover of a particular species of tree.

In JOTR, the desert willow grows to about 18'; with more water, it can grow up to thirty feet. Desert willow grows a very deep tap root, down to 50 feet! Desert willow is one of the few trees in the desert that is not a legume. Another name for the desert willow is mimbré or wicker, as some Native Americans made baskets from its branches.

Used extensively in landscaping throughout the southwest and as far north as the Texas Panhandle, the desert willow has long green narrow leaves similar to a willow, but it is not a willow. Flowers are quite variable from white to purplish red, pinkish white and bi-color, flowering in nature in May and June and into September with available water. The appearance of the flower has been compared to orchids. Leaves usually drop when the temperature descends into the low forties preceding winter.

The desert willow puts out numerous seedlings throughout the summer, usually sprouting the next year. I've dug up month-or-two old seedlings that can be easily transplanted with success, but it's best not to wait until the taproot is too long. The flower color of the seedlings will vary from the parent. Even if the taproot doesn't reach a local water source, after establishment, only monthly supplementation is required during the hottest months during periods of drought. Desert willows are often multi-trunked, but can be shaped to a single trunk. For a tidier appearance after leaf fall, snip off the seed pods.

Desert Lavender

Desert lavender, Hyptis emoryi Shrubland Alliance, forms stands along many of the minor washes (rather than larger ones) that are low, hot, narrow, rocky, sandy and very dry. Desert lavender is native in the Colorado, Sonoran and southern Mojave Deserts below 2800'. The desert lavender shrubland alliance can be found in areas of flood disturbance where leguminous trees aren't usually found.

Desert lavender grows up to 10 feet tall with gray spreading branches. Dense hairs cover the twigs. Its one inch leaves are gray to whitish and soft to the touch being covered in fine hairs. When brushed against and after rain, the leaves emit a minty or lavender fragrance; it is a member of the mint family. The flowers are small light violet and lavender in color, blooming more robustly March to May and less so throughout the year. Cold tolerance for desert lavender is most likely in the twenties, as it's northern range is southern Mojave.

In years of higher moisture, the leaves grow larger and with less hairs. It is used in landscaping in Zone 8 or warmer regions due to its attractive upright and compact form, fragrant leaves and flowers and long bloom season. Bees and hummingbirds frequent the flowers.

Chuparosa

Chuparosa (Justicia californica) has it's own shrubland alliance, though occupying a small portion of the park. I've included it here only because its showy red tubular flowers seemed like a flag waving one to stop the car and look at it and the surrounding plant alliance.

Cuparosa grows to 6 feet tall with inter-tangled stiff spreading branches that are often leafless or sparsely leafed with small gray-green leaves. It vigorously puts on one inch orange-red tubular flowers from March to June but can flower throughout the year; it is frost tolerant to 28º and freezes to the ground in the low 20's. Even though it is frost tolerant, it must not be very freeze tolerant as the lower elevations (not more than 2500 ft.) of JOTR is where it will be found, that is, in the Colorado Desert, rather than the Mojave. It's natural range is within the Sonoran Desert: Southern California, Southern Arizona, Baja California and in Mexico along washes and rocky slopes.

The Blue Palo Verde tree (Parkinsonia florida or Cercidium floridum, syn.) forms a Woodland Alliance with the desert ironwood (Olneya tesota) within Joshua Tree National Park in washes that experience flooding and have a higher water availability. It's referred to as a dry or xero-riparian species as it needs more moisture than what falls naturally. The “woodlands” form two strips of trees alongside the banks of broad, sandy washes. Blue palo verde is a multi-trunked tree with deep roots that can penetrate to the groundwater. Blue palo verde's natural range is up to 4,000 ft. along dry washes and on floodplains in Southern California, Arizona, Baha California and Sonora, Mexico.

The bark and twigs of young branches are blueish green (palo verde roughly means “green stick”) with small pinnately compound leaves that drop during the dry season and due to cool autumn temperatures. The blue-green bark performs the function of photosynthesis along with the tiny leaves. As the bark matures, it turns brown. Small spines can be found at the node up and down each branch.

The Blue Palo Verde is one of the showiest trees of the desert when it flowers – profusely for two weeks in mid spring. It glows with bright yellow flowers that nearly completely cover the tree. Many botanic gardens in Arizona feature the blue palo verde in their collections; a mid-March trip there will find it in glorious bloom. When in bloom, the Blue Palo Verde is a vibrant bee magnet, attracting numerous species of solitary, sweat, and leafcutter bees, bumblebees and carpenter bees, often at the same time, buzzing as it glows. (Photo at left from the Desert Botanic Garden, Phoenix).

It is said to be cold hardy to 15° and will do well in most soils as long as they're well drained. Leaf drop during drought can be prevented with good soakings once a month, if one is fortunate enough to live in a Zone 8 cold hardiness zone. The blue palo verde is the state tree of Arizona along with the foothill palo verde (Parkinsonia microphylla).

Smoketree

Appearing in patches in great enough densities, another tree forming a shrubland alliance in the greater wash system is the smoketree (Psorothamnus spinosus). The smoketree lives right in sandy and gravelly desert washes, and requires flooding to establish a stand. Smoketrees are relatively short-lived to about 50 years, so stands come into and out of existence regularly. Smoketree forms stands together with other shrubs and small trees and is more cold tolerant than the palo verde and desert ironwood, but less than the desert willow; cold damage appears at about 25°. The natural range of the smoketree is in gravelly and sandy washes up to 1500 ft. in the Colorado Desert of southern California, southwestern Arizona, Baja California and Sonora, Mexico.

Reaching a height of 10-15 feet, the smoketree drops its leaves soon after they appear giving the appearance of being leafless, as it is the majority of the time. Its slender gray branches are spiny. The smoketree is named for its gray new growth that looks like smoke from a distance. Small, deep violet-blue fragrant pea-like flowers appear on the smoketree in April-June. A smoketree can suffer considerable branch die off during periods of drought stress.

I encourage one to step out of the car and walk up and down the dry washes to view the plant community, as long as it's not raining upstream. Found growing in many smoketrees was the desert mistletoe (Phoradendron californicum), photo at right, whose masses of red berries caught our eye. If not for the mistletoe, I might not have hiked down the wash to see what it was. In addition to the smoketree, other leguminous trees are host to the desert mistletoe, including the catclaw acacia, honey mesquite, palo verde, desert willow and ironwood in JOTR.

Muller Oak

Another shrubland alliance within Joshua Tree National Park is the Muller Oak (Quercus cornelius-mulleri) Shrubland Alliance. Muller Oak, also called desert scrub oak, can also be found in certain areas of southern and Baja California. Muller oak, a Zone 8a shrub, grows along the piñon-juniper woodland, chaparral, dry slopes, alluvial fans, foothills and desert mountains from 4000-5200 ft. in elevation, the higher elevations of the park. Muller Oak is an evergreen shrub that grows to roughly 9-18 feet tall with thick, bicolor (gray-green with a yellow edges) leaves with whitish wool or hairs on the underside of the leaves. Acorns are produced irregularly and are eaten by bluejays, squirrels, chipmunks and other wildlife and cattle making the Muller oak a valuable plant within this ecosystem.

A hike through the Hidden Valley is a quick and pleasant walk to experience this oak shrubland among a scenic areas of low foothills accented throughout by boulders, a much different environment from the dry washes and flats. In this area, one can find pancake pear cactus (Opuntia chlorotica) and Parry's nolina (Nolina parryi) photo at right; some were conveniently beginning to bloom while we were there.

Teddy Bear Cholla

One of the more unusual shrublands in JOTR is that of the teddy bear cholla cactus. Driving along the park road, suddenly one finds oneself in the midst of a large stand. When backlit, these massings of teddy bear cholla give an eerie appearance. Stands of teddy bear cholla cactus form the Cylindropuntia bigelovii Shrubland Association (formerly Opuntia bigelovii). Within Joshua Tree National Park they occupy limited areas of low elevation to 1500-2100' in sandy and loam soils of good drainage. Teddy bear cholla stands can be dense, as they clonally reproduce.

This cholla cactus grows to about 5 feet in height from a single trunk that is densely branched or jointed. The 2 inch thick joints are thickly covered with 3/4” straw colored spines. The fuzzy appearance of the cholla give the appearance of a cuddly teddy bear, but don't be tempted to hug. Indeed be careful not to brush against them as the joints readily break off and are carried off, thus dispersing the cholla. Small, one inch greenish yellow flowers sometimes streaked with purple bloom from February to May in the Colorado, Mojave and Sonoran deserts. In viewing a stand of teddy bear cholla, one might think a wild fire swept across the desert, as the trunks and lower spines turn black as the cholla matures.

Desert Fan Palm

After driving along salt flats, plains, washes, desert lowland mountains, and the piñon-juniper woodlands, a visit to an oasis is a unique and welcome experience. Many different plants and animals inhabit this small area that won't be found throughout other desert alliances.

An uncommon woodland alliance I'm noting (this is not a complete list of JOTR Woodland and Shrubland Alliances) is that of the desert fan palm, Washingtonia filifera Woodland Alliance. There are 158 desert fan palm oases in North America and five are located in JOTR. Many palm oases are in southern California and in the Baja Peninsula.

The desert fan palm, Washingtonia filifera, is also called the California Fan palm. In JOTR, they are only found when fault lines have forced water up into fresh water springs. The desert fan palm, although requiring a constant supply of water, is considered a true desert plant as an oasis is a desert environment. The desert fan palm grows up to 75 feet tall. Its leaves are shaped like a fan and folded like an accordion.

As more of the water from the springs is used in development and agriculture, the number of palms are steadily decreasing. Faults can also shift, causing a change in the water supply. At the city of Twentynine Palms, California, for instance, where there may only have been 26 palms (a slight miscounting), there are far fewer palms naturally remaining today at the oasis.

Mohave Yucca

Known as the “other” yucca in JOTR is the Mojave yucca, Yucca schidigera, and forms its own Yucca schidigera Shrubland Alliance. The range of the Mojave yucca is similar to the Joshua Tree but extends much further south and in lower elevations, being more heat tolerant. Its range is below 5000' elevation from the Mojave Desert, Southern California mountains and valleys, Colorado Desert, and Sonoran Desert regions of California and in Nevada, Utah, Arizona, and to Baja California, Mexico.

Although both the Mojave yucca and the Joshua tree are arboresent (that is tree form that forms a woody trunk, either single or multi-trunked), it is not difficult to identify them correctly as their two forms are quite different. The Mojave yucca grows to 15' tall but without the many arms of the Joshua tree. And it's 2-5 ft. long yellow-green to blue-green leaves are much longer than the Joshua tree's. They have a sharp terminal spine and the leaf margins sport many curled white filaments. The dense and rounded flower stalk arises from near center of the leaves to just even or a little above the leaves, creamy white often tinged with light purple, blooms March to May.

Joshua Tree

In JOTR, ones natural tendency is to hurry the drive through the park to see the park's namesake, the towering Joshua trees, but its worth the wait to explore the other regions thoroughly before reaching this signature plant. For one, entering the park from the southern Cottonwood Springs Entrance allows for exploration of the lower elevations during the cooler morning hours. Secondly, the lower areas offer a great many plants of interest to the serious desert visitor.

And finally, the plant for which Joshua Tree National Park is name for, Yucca brevifolia, the indicator species of the Mojave Desert. TheYucca brevifolia Woodland Alliance grows over a range larger than just JOTR: in the Mojave Desert, southern California mountains and valleys, limited areas of the southeastern Great Basin regions of California and in southern Nevada, western Arizona, and southwestern Utah.

The Joshua Tree yucca grows in the northwestern corner of the park at higher and cooler elevations from 3700-4300 ft. on flats and gentle slopes, but in other regions grows from 2000-6000'. These higher elevations can be 3-10° cooler from lower salt flats and washes. The Joshua Tree yucca is slow growing and long lived and can withstand cold temperatures to 10º. Some sources stating cold hardiness to 0º or even lower, but not the very hot temperatures on the desert floor (up to 120°). Well drained soil is one key to growing in colder climates. The Joshua tree has deep roots extending down to over 30 feet.

The arborescent (tree-like) Joshua tree grows upwards to 30' and more with many thick spreading branches. The serrated leaves are rigid, sharply pointed, gray-green to blue green in color, usually growing 12-16” or so. As they mature and die, they hang down on the trunk. Although called a tree because of its appearance, it is not, and does not have growth rings, making dating the age difficult. The trunk grows wider with the years with a broken, bark-like appearance. The Joshua tree was used by Native Americans in many ways – leaves were woven into baskets and sandals, flower buds were eaten raw, seeds roasted and the wood used for building structures and as a fire source.

The Joshua tree yucca flowers from March to May. Clusters of creamy to greenish white flowers are arranged in a cluster at the tip of a branch. Current thinking is that freezing temperatures is required for branching and flowering. Flowering usually comes before branching (although branching can occur due to insect damage).

In driving through JOTR from the south to north, one first sees single Joshua trees here and there, as the elevation and boulders increase. Quickly, though, one soon sees Joshua tree woodland over the flat plains and rolling hills, and on into the mountains. This is not a dense canopy as one would experience in the Rocky Mountains or the Appalachians, but open and airy in appearance at the same time dense enough to be called a woodland.

Other Interesting Plants in Joshua Tree National Park

Even though many plants don't appear in a quantity or density to warrant their own plant alliance, or to be included among some of the higher density plants within an association, many are present in sufficient quantities to view as one drives through the park. Stopping along the frequent pull-outs provides the opportunity to more closely examine them and their habitat. It's hit and miss, especially to see them in flower on a single visit to the park. (Photo at left is brittlebush in the Colorado Desert just outside the part. Brittlebush does have its own plant alliance.) Not every acre was mapped and described by the vegetative survey, so new discoveries can still be made, and some plants may have been dormant during inspection by the survey teams.

Here are just a few of the other 750 documented plant species in JOTR, representing about 12% of California's total flora species and 33% of species within the California desert region (JOTR Plant Species List). In addition to trees, shrubs, woody and herbaceous perennials and many, many annuals, JOTR rich flora includes, bulbs, corms, mosses, liverworts, hornworts, cactus and other succulents, and lichens. Most of these are xeric, but a few are riparian plants, readily seen at the five oases. Some will be recognizable as members of genera of plants that are native in the Texas Panhandle or are familiar to us generally.

Lupines – several species of the Lupinus genus, best known in Texas as bluebonnets, although many flowers are magenta, purple or violet instead of blue.